I'm currently working with Goko on doing an overhaul that will clean up the "when-buy" / "when-gain" bugs, as well as getting Possession into a playable state so that Alchemy can be released. (By the way, Alchemy will probably be released before all the "when-buy"/"when-gain" bugs are fixed.)
One niggly bit that will actually require some substantial code change is playing Possession in a 1-player game. Although the official rules don't allow for 1-player games, the codebase supports it and you can even do it on the live server (just go to the "Secret Chamber" room). Most of the card rules generalize and work fine in 1-player games.
Possession is a bit of an exception. Technically, if you go with the letter of the rules, you should be able to play a turn where you are Possessing yourself. It would increment the round count differently, and the behaviour of cards you trash would be different on the Possessed turn.
Currently the implementation of Possession just ignores turns where you're Possessing yourself, which is clean and easy to code, and also means that internal routines can check quickly whether a turn is a Possession turn by checking to see if the controlling player and the current player are the same player. If we want a more correct implementation, a bunch of code needs to change, and I'm not sure whether it is worth the effort to make all these changes to support a mode that isn't in the official rules anyway (namely, 1-player games).
So I'm throwing up this poll. Please vote and I'll use the feedback to make a recommendation to Goko as to where to put this on their priority list (or if it's going to go there at all).